关系代数是以______为运算对象的一组高级运算的集合。

下列有关一氧化碳和二氧化碳的说法不正确的是                     [ ]

A. CO和CO₂都有毒，浓度高时都可以使人死亡

B. 温室里施用二氧化碳可提高农作物产量

C. 用澄清石灰水可区分CO、CO₂

D.一氧化碳具有可燃性，可用作气体燃料

根据课文内容用适当的词填空，使短文完整。(一空一词)

    Not long ago, a girl called Daisy took a magic 1.______ around the world, riding a

flying chair. First, she came to Tibet in China to visit antelopes. But to her great

2._____, she could only see mountains and people.

    Then an antelope told her 3.______ that they were being killed 4.______ large numbers

by humans for their wool to make clothing. And they might be 5._______ forever in three

years. Then the flying chair took Daisy to Zimbabwe, 6.______ wildlife was properly

protected. Daisy was glad to learn that farmers there were not killing the animals at

will as they 7._____ to. Only a certain number of the animals were set to be 8.______ for

fee by the government, 9.______ not only saved the animals but benefited the farmers as

well.

    At last, Daisy flew to a 10.______ rainforest. There she was puzzled to see a monkey

11.______ (rub) a millipede insect over its body. Then the monkey told her the insect

contained a 12._______ drug which affected mosquitoes and 13._______ rubbing it over the

body it could 14._______ itself from mosquitoes. Now Daisy was amazed and excited to find

in fact the forest 15._______ a lot of precious things people can make use of. So

protecting nature really means much to our humans.

在一定温度下，下列叙述不能作为可逆反应A（g）+3B（g）2C（g）达到平衡的标志的是                                    （   ）

①C的生成速率是B的生成速率的2/3倍  ②单位时间生成a mol A，同时生成3a mo1B ③A、B、C的浓度不再变化   ④混合气体的总压强不再变化

⑤混合气体的物质的量不再变化     ⑥单位时间消耗 a mol A，同时生成3amolB

⑦A、B、C 的分子数目比为1∶3∶2

A．②⑦              B．⑥④              C．①③              D．⑤⑥

下述关于数据库系统的叙述中正确的是

A．数据库系统减少了数据冗余

B．数据库系统避免了一切冗余

C．数据库系统中数据的一致性是指数据类型一致

D．数据库系统比文件系统能管理更多的数据

简述小型水库工程库址选择要注意的问题。